mubuntu,

IIRC postinst and postrm files need to have their digsigsums in a file in the DEBIAN folder too otherwise the system rejects them... I remember facing an issue of my postinst and prerm not being accepted by the system due to a lack of digsigsums.

Secondly, its never a good idea to be moving files with extension .bak and then moving them back when uninstalling the .deb; a better solution would be to have within your .deb two folders one for installing and one which contains the original files for restoring.

This way even if somebody has modified their system files they don't face any issues.